1. Field of the Invention
The present invention relates to a ball bearing, and particularly relates to a ball bearing used, for example, in a transmission portion of a car under the condition that lubricating oil with a high traction coefficient, such as automatic transmission fluid (hereinafter abbreviated to `ATF`), is used as a medium.
2. Description of the Related Art
As shown in the method of calculation for a dynamic rated load and a rated life of a rolling bearing in JISB1518, as for deep groove ball bearings and angular ball bearings, it is prescribed that the groove radius of an inner should not exceed 0.52 Dw and the groove radius of an outer race should not exceed 0.53 Dw where Dw designates the diameter of a ball, in the calculation of a basic dynamic radial rated load of the bearing.
This is because when the groove radius of a race is set to be larger than the above-mentioned value, the dynamic rated load is reduced so that the calculated rolling life is lowered. It is therefore a conventional solution to set the groove radius of a race to be as small as possible in order to reduce the contact surface pressure of the race surface to attain a long life of the bearing.
Lubricating oil superior in fluidity, such as spindle oil, turbine oil or the like, is used for lubrication of such a rolling bearing. On the other hand, gear oil, machine oil or the like is also used so that the lubricating oil for the rolling bearing is made common with lubricating oil for other parts around the bearing such as gears and so on.
Rolling bearings may be used under various environments. For example, in a radial ball bearing attached to a portion for guiding a gear or the like and supporting a rolling load, a slip is apt to be produced when lubricating oil such as ATF or the like having a high friction coefficient unsuitably for the rolling characteristic of the bearing is used as lubricating oil. It has been confirmed that flaking is produced in a race in a short time in accordance with the increase of the friction coefficient or the increase of the temperature.
ATF is lubricating oil used in an automatic transmission including a torque converter, a gear mechanism, a hydraulic mechanism, a wet clutch, etc. ATF is required to have many functions such as lubrication of a heating medium or a friction material, preservation of appropriate frictional characteristic, and so on, in order to actuate the mechanisms of the automatic transmission smoothly.
In a rolling bearing used under an environment using such ATF having a high traction coefficient, for example, in a radial ball bearing, a differential slip of an inner race which is a rotating race becomes large. As a result, the position where a Pv value (the product of contact surface pressure and velocity) becomes the maximum is located not at the center of the race (see FIG. 3) but at two places which are apart from the center in a contact ellipse (see FIG. 4), so that the tangential force increases. It was confirmed that flaking is produced in a shorter time than the calculated rolling life prescribed in JISB1518.
There is known a technique for changing the groove radius of a race of a rolling bearing so as to improve the bearing life as disclosed, for example, in Japanese Utility Model Uneximined Publication No. Hei. 5-86026. According to the technique disclosed therein, not only at least one of outer and inner races is formed from powdered high speed steel having a hardness of HRC 65 or more, but also a value obtained by dividing a ratio .rho. (R/r) of a groove radius R of the race to a rolling element radius r thereof by a ratio .rho..sub.0 (R.sub.0 /r.sub.0) of a groove radius R.sub.0 of a race formed from bearing steel to a rolling element radius r.sub.0 thereof is set to be in a range of from 1.0 to 1.12. As a result, torque or very small slips can be reduced so that the temperature of the bearing can be restrained from increasing. As a result, the life of the bearing can be improved.
There is also known a full type ball bearing in which the groove radius of an inner race (rotating race) is made larger than the groove radius of an outer race as disclosed in Japanese Patent Unexamined Publication No. Hei.7-23898. In the full type ball bearing, slips of balls can be reduced at the time of high temperature and high velocity while the influence of heat can be reduced, so that the rolling life can be prolonged.
However, in the technique disclosed in the Japanese Utility Model Uneximined Publication No. Hei. 5-86026, the cost of the bearing becomes high because at least one of the outer and inner races is formed from powdered high-speed steel having a hardness of HRC 65 or more. In addition, it is assumed that the bearing is used not under an environment in which ATF having a high traction coefficient is used but under an environment in which spindle oil having a high fluidity is used in a machine tool. Therefore, even if the groove radii of the races are simply changed, the temperature rising of the bearing is merely suppressed under the environment using ATF. Accordingly, this technique cannot be expected as measures against flaking.
On the other hand, in the technique disclosed in the Japanese Patent Unexamined Publication No. Hei.7-23898, the groove radius of the inner race (rotating race) is set to be 61.+-.2% of ball diameter Dw, while the groove radius of the outer race (fixed race) is set to be 56.+-.2% of the ball diameter Dw. Therefore, the contact surface pressure becomes small because the number of rolling elements is large in a full type ball bearing. However, in the case of a general deep groove ball bearing, for example, the value of the contact surface pressure Pmax reaches 450 kgf/mm.sup.2 or more when P (dynamic equivalent load)/C (JIS dynamic rated load)=0.5 and the groove radius of the rotating race is 59% of the ball diameter Dw. Thus, plastic deformation appears in the rolling race surface, so that it is difficult for the bearing to fulfill the function as a bearing.